Steam reforming of hydrocarbons

ABSTRACT

STEAM REFORMING IS EFFECTED TO PRODUCE A METHANE-RICH GASEOUS PRODUCT, SUITED FOR USE AS A SYNTHETIC NATURAL GAS, FROM HIGHER-BOILING HYDROCARBONS. A PORTION OF THE INITIAL REACTION ZONE PRODUCT EFFLUENT, GENERALLY FROM 3.0 MOL PERCENT TO ABOUT 50.0 MOL PERCENT IS REACTED IN A   SECOND REACTION ZONE AT HYDROGEN-PRODUCING CONDITIONS; THE PRODUCT THEREFROM IS RECYCLED TO THE INITIAL REACTION ZONE.

July lo, D, 1 WARD STEAM REFORMING OF HYDROCARBONS Filed April 26, i971 QN 111111 11 @Smm NQ.Q\

A"United States Patent Office 3,744,981 Patented July l0, 1973 3,744,981 STEAM REFORMING OF HYDROCARBONS Dennis I. Ward, South Barrington, Ill., assigner to Universal Oil Products Company, Des Plaines, Ill. Filed Apr. 26, 1971, Ser. No. 137,495 Int. Cl. C01b 2/14 U.S. Cl. 48-214 11 Claims ABSTRACT OF THE DISCLOSURE Steam reforming is effected to produce a methane-rich gaseous product, suited for use as a synthetic natural gas, from higher-boiling hydrocarbons. A portion of the initial reaction zone product efduent, generally from 3.0 mol percent to about 50.0 mol percent is reacted in a second reaction zone at hydrogen-producing conditions; the product therefrom is recycled to the initial reaction zone.

APPLICABILITY OF INVENTION The present invention involves the reforming of hydrocarbonaceous material, in the presence of steam, to produce lower-boiling products. More specically, the present invention is directed toward hydrocarbon steam reforming to produce a methane-rich gaseous product particularly well suited for utilization as a synthetic natural gas (SNG), often referred to as Town Gas. The process is effected catalytically, preferably utilizing a fixed-bed reaction zone through which the various reactants are passed.

It is Well known that the steam reforming of hydrocarbons, utilizing normally gaseous material, or normally liquid naphtha fractions, can be effectively and eiciently utilized for the production of gaseous products suitable for chemical syntheses, or combustion as town gas. A great deal of interest has recently developed in the area of atmospheric pollution resulting from the combustion of various fuels for a multitude of purposes. In effect, the result has been a gradual, but increasing reluctance to utilize high sulfur-containing coal and fuel oils, with more reliance being placed upon the use of natural gas rich in methane. The ever-increasing demand for greater quantities of natural gas has brought about a critical and sudden depletion in the natural resources thereof. Maintaining the current rate of natural gas use, while recognizing that the rate is steadily increasing, will, according to those having eX- pertise, result in a total dep-letion of the natural gas reserve within a period of about seven to ten years.

To alleviate such an adverse situation, it is foreseen that more and more petroleum refiners, as Well as gas producers, will turn to the relatively ancient technology of hydrocarbon steam reforming. The requirement for voluminous quantities of methane-rich gas Will give rise to many innovations in reforming technology, most of which will be centered around improved catalytic composites and processing techniques designed to afford an extended period of operation While producing maximum yields of synthetic natural gas. lIt is to this end that the various objects and embodiments of the present invention are particularly directed.

OBJECTS AND EMBODIMENTS A principal object of the present invention is to improve the efficiency of a process for effecting the steam reforming of hydrocarbons. A corollary objective resides in extending the period of time during which the process functions acceptably and economically.

Another object of my invention is to provide a processing technique which decreases the extent to which carbon becomes deposited on the catalyst employed in a hydrocarbon steam reforming process.

Therefore, in a broad embodiment, the present invention involves a steam reforming process which comprises reacting a hydrocarbon and steam, in a first reaction zone, at steam reforming conditions; further reacting a portion of the resulting first zone etiluent, in a second reaction at hydrogen-producing conditions; and, recycling the resulting second zone effluent to said rst reaction zone.

A more limited embodiment of my invention is directed toward a steam reforming process which comprises reacting steam and a normally liquid naphtha charge stock, in a first reaction zone, at conditions seletced to convert said charge stock into methane, including a temperature in the range of about 800 F. to about 1100 F.; further reacting a portion of the resulting first zone eiiluent in a second reaction zone, at hydrogen-producing conditions including a temperature from l F. to about 1500 F.; recycling the resulting second zone eliluent to said first reaction zone; and, recovering a methane-rich product from the remaining portion of said first zone eiiluent.

Other objects and embodiments relating to the present inventive concept will become evident from the following additional description of the process. In one such other embodiment, steam is introduced into said second reaction zone in admixture with the portion of the first zone eiiluent.

SUMMARY OF INVENTION As hereinbefore set forth, the present invention encompasses a process for the catalytic conversion of hydrocarbons through the reforming thereof in the presence of steam. The principal function of the present process is the production of normally gaseous material, and particularly a methane-rich end product. Suitable charge stocks, from which high yields of methane will be obtained, include normally gaseous components such as ethane, propane and butane; a normally liquid light naphtha having an end boiling point in the range of about 250 to about 300 F.; and a normally liquid heavy naphtha having an initial lboiling point of about 250 to about 300 F. and an end -boiling point of about 400 F. to about 450 F. Another suitable charge stock Would be a mixture of both normally gaseous and normally liquid componentseg a light straight-run naphtha containing, ethane, propane and butane.

As is well known in the prior art, the greater proportion of suitable steam reforming catalytic composites are sensitive to the presence of sulfurous compounds in the charge stock, and are knovvn to deactivate rapidly as a result. In the following discussion, therefore, it will be presumed that the charge stock to the present process hasl previously been subjected to some form of hydrotreating, or hydrorefining in order to convert the sulfurous compounds into hydrogen sulfide and hydrocarbons, and that the resulting hydrogen sulfide has been removed prior to being charged to the present process. In short, suitable charge stocks for the present process should contain less than about 25 ppm. by weight of sulfurous compounds, and preferably less than about 10.0 ppm., calculated as elemental sulfur. One particular suitable hydrorening pretreatment involves the use of a cobalt-molybdenum catalyst at a maximum catalyst bed temperature in the range of 600 F. to about 850 F. Other operating conditions include a pressure of from 250 p.s.i.g. to about 1500 p.s.i.g., a liquid hourly space velocity of 0.1 to about 10.0 and a hydrogen concentration of about 100 to about 1500 s.c.f./ bbl. The resulting hydrogen sulfide may be removed in any suitable manner including stripping, adsorption over a zinc oxide adsorbent, etc. -It is understood that the hydrorefining pretreatment forms no essential part of my invention, and any suitable technique for reducing the sulfur content to less than about 25.0 ppm. by Weight will suffice.

The substantially sulfur-free charge stock is admixed with steam in an amount to result in a steam/carbon ratio in the range of about 1.1 to about 6.0, and preferably from about 1.3 to about 4.0. The mixture is passed into a steam reforming reaction zone at a temperature such that the maximum catalyst bed temperature is in the range of about 800 F. to about 1100 F., and preferably from about 825 F. to about 1000"' F. The steam reforming reactions will be effected at an imposed pressure in the range of about 250 p.s.i.g. to about 1500 p.s.i.g., and preferably from about 400 p.s.i.g. to about 1000 p.s.i.g. A Wide variety of steam reforming catalytic composites are well known, and have been thoroughly described in the prior art. In general, these catalysts utilize metallic components selected from Group VI-B and the iron-group of the Periodic Table, including chromium, molybdenum, tungsten, nickel, iron and cobalt. Also thoroughly disclosed within the prior art are the benefits to be accrued through the utilization of catalytic promoters selected from alkali and alkaline-earth metals, including lithium, sodium, potassium, rubidium, beryllium, magnesium, calcium, strontium and barium. These catalytic components are generally combined with a suitable refractory inorganic oxide carrier material, either synthetically-prepared or naturally occurring. Suitable refractory inorganic oxides material includes kieselguhr, kaolin, attapulgus clay, alumina, silica, zirconia, hafnia, boria, etc., and mixtures thereof. One particularly suitable and preferred steam reforming catalyst is that described n U.S. Pat. No. 3,429,680 (Cl. 48-214), which catalyst utilizes a carrier material of kieselguhr and a catalytically active nickel component promoted through the use of a copper-chromium, or copper-chromium-man- Iganese complex, and may, or may not be further promoted by the addition of an alkaline-earth metal` oxide. This particular catalyst is preferred since it appears to possess an unusually high degree of sulfur tolerance. The reaction zone product eiuent, principally comprising methane, carbon monoxide, carbon dioxide, hydrogen and steam is cooled to a temperature in the range of about 400 F. to about 800 F., preferably with an upper limit of about 650 F. A portion of the cooled product effluent, generally from about 3.0 mol percent to about 50.0 mol percent, and preferably up to about 20.0 mol percent, is diverted to a second reaction zone functioning at substantially the same pressure, but at an elevated temperature in the range of about 1100 F. to about 1500 F. In a preferred embodiment, up to about 50.0% of steam, based upon fresh feed charge stock, is admixed with the portion of the eiuent being diverted to the second reaction zone.

The catalytic composite in the second zone may be the same as that disposed in the first reaction zone, and is generally selected from those catalysts hereinbefore described. Preferably, however, the catalyst used in the hydrogen-producing reaction zone is an iron-group metal component combined with a refractory inorganic oxide such as a composite of alumina and silica. At the elevated severity of operation, hydrogen-producing reactions are effected with the result that the hydrogen concentration is increased from a level of about 20.0 mol percent to a level in the range of from about 40.0 mol percent to about 60.0 mol percent, on an essentially dry basis. The hydrogen-enriched gaseous phase is then recycled to combine with the charge to the initial reaction zone. A methane-rich gas is separated and recovered from the remaining portion of the lirst reaction zone eluent. A particularly preferred system for recovering the desired end product is that shown in the accompanying drawing.

DESCRIPTION OF DRAWING In the drawing, the embodiment presented is illustrated as a simplified ow diagram in which such details as pumps, instrumentation and controls, heat-exchange and heat-recovery circuits, start-up lines, compressors, valving and similar hardware have been omitted as being non-essential to an understanding of the techniques involved. The utilization of such miscellaneous appurtenances, to modify the process, are well within the purview of those possessing expertise in the art of petroleum reiining technology.

For the purpose of demonstrating the illustrated ernbodiment, the drawing will be described in connection with the steam reforming of a light straight-run naphtha. containing propane, butane and pentane. It is understood that the charge stock, stream compositions, operating conditions, separators, reactors and the like are exemplary only and may be varied widely without departure from the spirit of my invention, the scope of which is defined by the appended claims.

The drawing will be described in conjunction with a commercially-scaled unit designed to process approximately 6,273 bbls./day of the light straight-run naphtha. The charge stock contains 221 barrels per day of propane, 607 barrels per day of butanes, 1,280 barrels per day of pentanes and 4,165 barrels per day of hexanes and heavier normally liquid hydrocarbons. The charge stock enters the process through line 1, and is admixed therein with 6,910 mols/hr. of steam, from line 2, resulting in a steam to carbon ratio of 1.6. The mixture continues through line 1 into heater 3, wherein the temperature is raised to a level such that the temperature at the inlet to the reactor is about 930 F. The heated stream passes through line 4, and is admixed with a hydrogen-rich recycle stream from line 5, the source of which is hereinafter described, the mixture continuing through line 4 into reactor 6. Reactor 6 contains about 1,010 cubic feet of a catalytic composite having an apparent bulk density of about 0.98 gram per cc. The catalyst comprises a carrier material of kieselguhr, about 38.0% by weight of a nickel component (calculated as elemental nickel), about 9.0% by weight of magnesium oxide and about 7.5% by weight of a copper-chromium-manganese component in which the mol ratio of copper to chromium to manganese is 1.0: 1,010.1. The pressure imposed upon reactor 6 is about 590 p.s.i.g., as measured at the inlet thereto.

The product efiiuent from reaction zone 6 is Withdrawn by way of line 7 and introduced into condenser 8, wherein the temperature is reduced to a level of about 520 F., the cooled eiuent being withdrawn by way of line 9. The total product etiluent from reaction zone 6 has the approximate composition indicated in the following Table I:

TABLE I Steam reforming product eiiiuent Component: Mol percent Methane 32.2 Carbon monoxide 0.5 Carbon dioxide 11.5 Hydrogen 8.6 Steam 47.2

About 30.0 mol percent of the cool product effluent from line 9 is diverted by way of line 24 and, after being compressed to a level of about 750 p.s.i.g., continues through line 24 into heater 25 wherein the temperature is increased to a level of about 1200 F., the heated material passing through line 26 into reactor 27. In this particular illustrtion, reactor 27 has disposed therein a catalytic composite of 15.0% iron, calculated as the element, combined with a composite of 63.0% alumina and 37.0% by Weight of silica. The effluent from reactor 27 is withdrawn by way of line 5 and recycled to combine with the heated charge stock and steam in line 4.

The remaining portion of the product effluent in line 9 continues therethrough into shift converter 10, at a temperature of about 520 F. and at substantially the same pressure. The product etliuent from shift converter 10, at a temperature of about 600 F., is withdrawn by way of TABLE II.SHIFT CONVERTER EFFLUENT Converters Component, molslhr. l0 14 Methane 3, 334. 8 3, 359. 0 Carbon monoxide 1.3 0.1 Carbon dioxide 982. 4 959. 4 Hydrogen 133. 3 37. 7 Steam 4, 943. 4 4, 990. 6

The product eliiuent from second shift reactor 14, at a temperature of about 520 F. and a pressure of about 515 p.s.i.g., is withdrawn through line into condenser 16, wherein the temperature decreased. The cooled eiiiuent is introduced via line 17 into a suitable separator 1.0, from which condensed Water is removed from the process by way of line 19. The eiiiuent from shift converter i4, now substantially free from water, is introduced by Way of line Z0 into carbon dioxide removal system 2l. Carbon dioxide is removed via line 22, while a methane-rich product gas is recovered in line 23.

In Va preferred embodiment, as hereinbefore stated, steam is diverted from line 2 through line 23, to combine with the iirst zone eflluent in line 24. In the illustration, about 20.0% of steam, based upon the fresh feed in line 1, is so diverted.

The removal of carbon dioxide in system 21 may be effected in any manner Well-known to the prior art. One such conventional manner involves mono-ethanolamine adsorption. Another adsorption scheme utilizes hot potassium carbonate, while another suitable technique employs a catalytic reaction system utilizing vanadium pentoxide as the catalyst. The final methane-rich gaseous product in line 23 has the composition indicated in the following Table The principal advantage to be obtained through the utilization of the present invention stems from the increase in molecular hydrogen concentration in the total charge to gasification reactor 6. The effective catalyst life, expressed as barrels of normally liquid charge stock per pound of catalyst disposed in the reaction zone, will be increased from 25.0% to about 45.0%. Other advantages include a smaller charge heater 3, signjiicantly more ethcient overall heat utilization and a more isothermal gasification reactor.

'The foregoing specification clearly illustrates the method of etfecting the present invention and the benefits to be alforded through the utilization thereof in the steam reforming of hydrocarbons for the purpose of producing a methane-rich gaseous product.

l claim:

i. A method for producing a methane-rich gaseous product which comprises reacting a normally vaporous or normally liquid hydrocarbon charge stock and steam, in a rst catalytic reaction zone, at conditions selected to convert the charge stock` into methane; further reacting a portion of the resulting iirst zone etliuent, in a second catalytic reaction zone, at hydrogen-producing conditions; and, recycling the resulting second zone eiiiuent to said first reaction zone; and, recovering a methanerich product from the remaining portion of said first zone eiiiuent.

2. The process of claim i further characterized in that said steam reforming conditions include a temperature in the range of about 800 F. to about 1,025 F., a pressure from 250 p.s.i.g. to about 1,500 p.s.i.g` and a steam/ carbon ratio from about 1.1 to about 6.0.

3. The process of claim l further characterized in that said hydrogen-producing conditions include a temperature in the range of about 1,100o F. to about 1,500o F.

4. The process of claim l further characterized in that from about 3.0 mol percent to about 50.0 mol percent of said tirst zone effluent is reacted in said second reaction zone.

5. The process of claim i further characterized in that steam, in an amount up to 50.0% based upon said hydrocarbon, is introduced into said second reaction zone.

6. The process of claim l further characterized in that said hydrocarbon contains at least two carbon atoms per molecule.

7. The process of claim 6 further characterized in that said hydrocarbon is normally vaporous.

8. The process of claim d further characterized in that said hydrocarbon is normally liquid and boils in the light naphtha boiling range.

9. The process of claim 6 further characterized in that said hydrocarbon is normally liquid and boils in the heavy naphtha boiling range.

lil). A method for producing a methane-rich gaseous product which comprises reacting steam and a normally liquid naphtha charge stock, in a first catalytic reaction zone, at conditions selected to convert said charge stock into methane, including a temperature in the range of 800 F. to about ll00 F.; further reacting a portion of the resulting first zone effluent, in a second catalytic reaction zone, at hydrogen-producing conditions including a temperature from '1,100 F. to about 1,500" F.; recycling the resulting second zone eiiiuent to said first reaction zone; and, recovering a methane-rich product from the remaining portion of said tirst zone etiiuent.

1l. The process of claim 10 further characterized in that said iirst and second reaction zones contain a catalytic composite comprising a porous carrier material and an iron-group metal component.

Reierences Cited UNITED STATES PATENTS 2,840,462 6/1958 Gorin 48-202 X 3,115,394 12/1963 Gorin et al 48-202 X 2,605,176 7/1952 Pearson 48-214 3,420,642 1/ 1969 Percival 48-214 3,459,520 8/1969 Percival 48--214 MORRIS O. WOLK, Primary Examiner R. E. SERWIN, Assistant Examiner US. Cl. X.R. 48-197 R, 213 

